Ca-La-F BASED TRANSPARENT CERAMIC, Ca-La-F BASED TRANSPARENT CERAMIC, OPTICAL ELEMENT, OPTICAL SYSTEM, AND CERAMIC-FORMING COMPOSITION

ABSTRACT

A Ca—La—F based transparent ceramic, including: mixing CaF 2  particles and LaF 3  particles that are prepared separately from the CaF 2  particles to form a mixed body of particles, and sintering the mixed body of particles and making the mixed body transparent, thereby producing a transparent ceramic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.12/801,512 filed on Jun. 11, 2010, which is a continuation ofInternational Application No. PCT/JP2008/072696 filed on Dec. 12, 2008,and published as WO 2009/075361, which claims priority to Japanesepatent application No. 2007-322437 filed on Dec. 13, 2007. The contentsof the aforementioned applications are incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to a method of producing a Ca—La—F basedtransparent ceramic that includes fluorides of calcium and lanthanum, aCa—La—F based transparent ceramic, an optical element, an opticalsystem, and a ceramic-forming composition.

Fluorite (CaF₂) has a high Abbe's number (vd) of 95, is excellent inrefractive index dispersion, and shows small dispersion of refractiveindex for different optical wavelength. Further, fluorite has hightransmittance of light ranging from ultraviolet region to infraredregion. Therefore, the fluorite is known as an excellent opticalmaterial.

It is possible to correct chromatic aberration satisfactorily bycombining a convex lens made of an optical material of theabove-described properties and a concave lens made of a differentmaterial. Therefore, the fluorite is frequently used in various opticalsystems, for example, objective lenses of microscopes (e.g., JapaneseUnexamined Patent Application, First Publication, No. 2004-191933).

Single crystalline fluorite has been conventionally utilized in opticalsystems. On the other hand, a method of producing a fluorite ceramic asa sintered calcium fluoride is known, for example, by JapaneseUnexamined Patent Application, First Publication, No. 2006-206359.Japanese Unexamined Patent Application, First Publication, No.2006-206359 describes the production method of fluorite ceramic asfollows. A suspension is obtained by a reaction of a calcium compoundand a fluorine compound in a solution. Next, fine particles of calciumfluoride are produced by loading the suspension in a closed vessel andheating the suspension at a temperature of not lower than 100° C. andnot higher than 300° C. A sintered body is formed by heating andsintering the calcium fluoride particles at a temperature of not lowerthan 700° C. and not higher than 1300° C. By heating the sintered bodyat a temperature of not lower than 800° C. and not higher than 1300° C.while pressurizing the sintered body by a pressure of not lower than500Kg/cm² and not higher than 10000 Kg/cm² in an inert atmosphere, thesintered body becomes transparent, thereby forming a fluorite ceramic.

Since the thus produced ceramic is a dense sintered body in whichoccurrence of voids or the like is suppressed, it is possible to achieveexcellent optical properties.

In a single crystalline fluorite which has been conventionally used inoptical systems, anisotropic strain is generated due to difference inthermal expansion in different crystallographic orientation at a time ofincreasing a temperature of the fluorite. Therefore, imaging propertiesof a lens tend to deteriorate due to thermal expansion strain generatedby fluctuation of ambient temperature. Further, the single crystallinefluorite is inferior in processability because cracking is easilygenerated by an abrupt change of temperature.

Although the fluorite has a high Abbe's number, refractive index (n_(d))of fluorite is a very low 1.43. Therefore, even when the fluoriteceramic is used, the optical applicability of fluorite tends to berestricted.

Although a Ca—La—F based crystal having a cubic crystal structure isknown, it is difficult to produce a homogeneous Ca—La—F based crystal ofhigh crystallinity stably. Therefore, it has been difficult to utilizethe Ca—La—F based crystal as an optical material.

SUMMARY

Based on the consideration of the above-described problems, an objectaccording to an aspect of the present invention is to provide a Ca—La—Fbased transparent ceramic and optical elements utilizing the transparentceramic, the ceramic having a high Abbe's number like that of fluorite,a refractive index higher than fluorite, and a transmittance sufficientfor utilizing the ceramic as an optical material. Another objectaccording to an aspect of the present invention is to provide an opticalsystem utilizing such an optical element.

Still other object according to an aspect of the present invention is toprovide a production method that enables production of a Ca—La—F basedceramic having an Abbe's number as high as that of fluorite and arefractive index higher than fluorite.

Still other object according to an aspect of the present invention is toprovide a composition that can be satisfactorily used in production ofthe above-described Ca—La—F based transparent ceramic.

According to a first aspect of the present invention, a Ca—La—F basedtransparent ceramic is constituted of a polycrystalline material thatincludes crystals of (Ca_(1−x)La_(x))F_(2+x), where x denotes a numberlarger than 0 and not larger than 0.4. The ceramic has a transparencythat enables transmission of light.

According to a second aspect of the present invention, an opticalelement includes the above-described Ca—La—F based transparent ceramicand is worked to have a predetermined shape.

According to a third aspect of the present invention, an optical systemcomprises at least a pair of a convex lens and a concave lens in anoptical path, wherein one of the pair of the convex lens and the concavelens is made of the Ca—La—F based transparent ceramic, and the other ofthe pair is made of a material different from the Ca—La—F basedtransparent ceramic.

The above-described optical system may further comprise one or moreconvex lenses, and one or more concave lenses.

According to a fourth aspect of the present invention, a method ofproducing a Ca—La—F based transparent ceramic includes, mixing CaF₂particles and LaF₃ particles that are prepared separately from the CaF₂particles to form a mixed body of particles, and sintering the mixedbody of particles and making the mixed body transparent, therebyproducing a transparent ceramic.

The above-described production method may include producing the CaF₂particles and/or producing the LaF₃ particles.

The above-described production method may include preparing a mixed bodyof particles including the CaF₂ particles and the LaF₃ particles.

According to a fifth aspect of the present invention, a ceramic-formingcomposition (a ceramic raw material composition) includes CaF₂ particlesand LaF₃ particles prepared separately from the CaF₂ particles.

A Ca—La—F based transparent ceramic according to an aspect of thepresent invention is constituted of a polycrystalline material including(Ca_(1−x)La_(x))F_(2+x) crystals (x denotes a number larger than 0 andnot larger than 0.4) and has transparency capable of transmitting light.Therefore, the Ca—La—F based transparent ceramic according to an aspectof the present invention has optical properties different from a CaF₂crystal and LaF₃ crystal. That is, according to an aspect of the presentinvention, it is possible to provide a Ca—La—F based ceramic having anAbbe's number as high as that of fluorite and a refractive index higherthan that of fluorite. In addition, since the Ca—La—F based transparentceramic is a polycrystalline material, there is an additional advantagethat isotropic thermal expansion strain tends to be generated at thetime of increase of temperature.

Since the optical element according to an aspect of the presentinvention includes the above-described Ca—La—F based transparentceramic, the optical element has a high Abbe's number like that offluorite, and has a refraction index higher than that of fluorite.Further, in the optical system according to an aspect of the presentinvention including the above-described optical element, it is easy torealize excellent optical performance.

In addition, in the method of producing a Ca—La—F based transparentceramic of an aspect of the present invention, CaF₂ particles and LaF₃particles prepared separately from the CaF₂ particles are mixed to forma mixture of the particles, and the mixture is sintered and become atransparent body. Therefore, it is easy to ensure a sinterability of theparticle mixture, and it is possible to produce a Ca—La—F basedtransparent ceramic that is composed of a polycrystalline body includingfluorides of calcium and lanthanum, and has a transparency that enablesoptical transmission.

Further, since the ceramic-forming composition according to an aspect ofthe present invention includes CaF₂ particles and LaF₃ particlesprepared separately from the CaF₂ particles, the composition can besatisfactorily used in the method of producing a Ca—La—F based ceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a TEM photograph of CaF₂ particles prepared in Example 1.

FIG. 1B is a TEM photograph of LaF₃ particles prepared in Example 1.

FIG. 2A is a chart showing a result of an X-ray diffratometory of aCa—La—F based ceramic produced in Example 1.

FIG. 2B is a chart showing a result of X-ray powder diffraction of aconventional CLF crystal.

FIG. 3 is a graph showing optical wavelength versus transmittance oflight in Ca—La—F based ceramics produced in Example 1 and Example 4. Thethick solid line A denotes a Ca —La—F based ceramic produced in Example1 wherein dispersion of particles in alkaline liquid was performed. Thethin solid line B denotes a Ca—La—F based ceramic produced in Example 4wherein dispersion of particles in alkaline liquid was not performed.

FIG. 4 is a graph showing changes of a refractive index and Abbe'snumber in accordance with change of a mixing ratio of CaF₂ particles andLaF₃ particles in the Ca—La—F based transparent ceramic produced inExamples 1 to 3.

FIG. 5 is a graph showing a correlation of Abbe's number and a partialdispersion ratio in Ca—La—F based transparent ceramics produced inExamples 1 to 3 and various optical glasses.

FIG. 6 is a graph showing wavelength-dependent transmittance of light inthe Ca—La—F based transparent ceramic produced in Example 5.

DESCRIPTION OF EMBODIMENTS

Hereafter, embodiments according to some aspect of the present inventionare explained.

A Ca—La—F based transparent ceramic of an embodiment of the presentinvention is substantially composed of polycrystalline body of calciumand lanthanum fluoride, and has a transparency capable of transmittinglight. The Ca—La—F based transparent ceramic can be used as a materialtransmitting light therein. Specifically, the transparent ceramic can beused satisfactorily as a material of an optical element such as a lens.

The Ca—La—F based ceramic is a polycrystalline body that includescrystals (a plurality of crystals) of calcium-lanthanum fluoride(hereafter, referred to as CLF). The CLF has a composition ratio (atomicratio) shown by a formula of (Ca_(1−x)La_(x))F_(2+x), where x denotes anumber larger than 0 and not larger than 0.4. The composition ratio canbe measured at high precision by X-ray fluorescent analysis or byvarious chemical analysis.

The composition ratio of CLF can be set at various values by, forexample, controlling the production conditions. It is preferable tocontrol the composition ratio of CLF such that the La content is nothigher than the solid-solubility limit of La in Ca-rich phase under thesintering conditions. In this case, it is possible to achieve astructure effectively dominated by a crystal structure of Ca-rich phasewhere end member is a CaF₂.

While CaF₂ belongs to a cubic crystal system and LaF₃ belongs to ahexagonal crystal system, CLF having a composition of theabove-described range is a cubic crystal. Where the crystal system iscubic, it is easy to maintain matching of crystal structure at the grainboundary. Therefore, where the CLF crystals have a cubic structure,there is a great advantage in making the ceramic transparent.

Where x of the above-described CLF composition shown by(Ca_(1−x)L_(x))F_(2+x) exceeds 0.4, La-rich portions tends to begenerated locally, thereby making it difficult to obtain ceramic havinghomogeneous optical properties. Therefore, the upper limit of x was setat 0.4. Preferably, x is controlled to be not lower than 0.1 and nothigher than 0.4. Where x is less than 0.1, it is impossible to achieve arefractive index remarkably larger than that of fluorite. Preferably, xis controlled to be not higher than 0.3 so as to prevent generation ofportions of high La concentration, and to stably achieve a homogeneousoptical properties. Therefore, it is more preferable to control x to benot lower than 0.1 and not higher than 0.3.

It is desirable that the Ca—La—F based transparent ceramic is composedsubstantially of CLF crystals. However, CaF₂ crystals and/or LaF₃crystals may be included in the ceramic provided that the ceramic has adesired refractive index and Abbe's number, and a transparencysufficient for an application of the ceramic as an optical material.

Further, inevitable impurities, sintering aids or the like may beincluded in the Ca—La—F based transparent ceramic provided that theceramic has a desired refractive index and Abbe's number, and atransparancy sufficient for an application of the ceramic as an opticalmaterial.

Preferably, the Ca—La—F based transparent ceramic does not contain theCaF₂ crystals and the LaF₃ crystals and is composed substantially of CLFcrystals. Preferably, the Ca—La—F based transparent ceramic is composedof substantially homogeneous CLF crystals. For example, it is preferredthat a fluctuation of x of the composition formula(Ca_(1−x)La_(x))F_(2+x) of the CLF crystal is suppressed within a rangeof the target value ±10% in a portion of 1 μm×1 μm×1 μm.

The Ca—La—F based transparent ceramic has a transparency capable oftransmitting light. The transparency may be at a level sufficient forintended use of the Ca—La—F based ceramic. For example, when the ceramicis used to transmit light of a certain wavelength, transmittance oflight of the wavelength may be 50% or more. When the Ca—La—F basedceramic provided by an embodiment of the present invention is applied asan optical element used in correction of chromatic aberration, theworking wavelength of the light may be, for example, in a range of 380nm to 780 nm, and a representative wavelength may be, for example, 550nm. Light of wavelength of 550 nm or less is used in various opticalelements. Therefore, Ca—La—F based transparent ceramic may be controlledto have a transmittance of 50% or more for the light of 550 nm inwavelength, and more preferably, a transmittance of 50% or more for thelight of 350 nm or more and 550 nm or less in wavelength. Preferably,the transmittance of light of the above-described wavelength orwavelength range is 70% or more, and more preferably 80% or more.

The transparency is achieved by specifying the material of the ceramicto fluoride of Ca and La. For example, even though alternative materialssuch as fluoride of Ce or Y may constitute a material having a highrefractive index, sufficient transparency cannot be achieved by usingsuch materials.

The above-described Ca—La—F based transparent ceramic may be used as itis, or may be used as an optical element having a predetermined shape.For example, the Ca—La—F based transparent ceramic may be processed toan optical element in which a surface of light incidence and/or asurface of light emission has various shape such as spherical shape,aspherical shape, planer shape, grating shape, or the like. Further,one, two, or more optical elements made of the Ca—La—F based transparentceramic may be used in combination. The above-described optical elementmay be use in combination with an optical element made of a differentmaterial in an optical system. For example, at least an optical elementmade of the Ca—La—F based transparent ceramic may be used in combinationwith at least one optical element made of a material selected from anoptical glass, an optical plastics, an optical crystal or the like.

The above-described Ca—La—F based ceramic is a novel ceramic made of asintered body of CLF crystals and is provided with a satisfactorytransparency. The ceramic and an optical element formed of the ceramicare fluoride materials having optical properties different from CaF₂crystal and LaF₃ crystals, and a refractive index and Abbe's numberdifferent from conventional materials. Specifically, the ceramic has ahigh Abbe's number similar to that of fluorite and a refractive indexhigher than that of fluorite. For example, the Ca—La—F based transparentceramic may has a refractive index (nd) of not lower than 1.43 and nothigher than 1.55, and an Abbe's number of not lower than 80 and nothigher than 95. Therefore, the ceramic can be easily utilized in variousoptical systems.

Further, as shown in the below-described FIG. 4 related to Example 4,the refractive index and the Abbe's number have a linear functionalcorrelation with proportions (abundance ratio) of Ca and La. Therefore,a ceramic having a desired refractive index and Abbe's number can beobtained easily by controlling the proportions of Ca and La.

Compared with various normal glasses (general optical glasses), theCa—La—F based transparent ceramic shows anomalous partial dispersion.For example, as shown by FIG. 5 related to the below-described Examples,correlation between Abbe's number (v_(d)) and partial dispersion ratio(Pg, F) in normal glass shows a distribution which can be approximatedby a straight line. On the other hand, in the case of Ca—La—F basedceramic, partial dispersion ratio plotted against the Abbe's numberdeviates from the above-described straight line, and shows adistribution different from normal glasses. Specifically, the Ca—La—Fbased transparent ceramic according to an embodiment of the presentinvention may have an Abbe's number of not lower than 80 and not higherthan 95, and a partial dispersion ratio (Pg, F) shown by the belowdescribed formula (1) of not lower than 0:53 and not higher than 0.56.Therefore, in the Ca—La—F based ceramic, a refractive index with respectto light of specific wavelength, for example, the Fraunhofer's g-linecan be different from normal glasses.

Pg,F=(ng-nF)/(nF-nC)   (1),

where ng, nF, nC denote refractive index regarding Flaunhofer's g-line,F-line, and C-line, respectively.

Therefore, color correction (correction of chromatic aberration) and/orremoval of secondary spectrum may be easily performed by utilizing theabove-described anomalous partial dispersion, and by combining anelement made of the Ca—La—F based ceramic and different elements. Forexample, in a case of an optical system equipped with a convex lens anda concave lens in the optical path, it is possible to remove a secondaryspectrum effectively by constituting the optical system using an elementmade of the Ca—La—F based transparent ceramic as one of the convex lensand the concave lens, and using an element made of a material differentfrom the Ca—La—F based transparent ceramic as the other one of thelenses. For example, the above-described optical system may be utilizedin a telephoto lens having a long focal length.

For example, a convex lens or a concave lens may be produced using theCa—La—F based transparent ceramic according to an embodiment of thepresent invention by processing a surface of the Ca—La—F basedtransparent ceramic to a predetermined shape. The Ca—La—F basedtransparent ceramic may be roughly shaped or molded to have a shapesimilar to the target product preliminary. After the processing, thesurface may be further subjected to opto-polishing. Where necessary, thesurface may be subjected to anti-reflection finishing or the like. Theabove-described processes may be performed by a known-method.

Since the Ca—La—F based transparent ceramic is a polycrystalline body,thermal expansion strain generated in accordance with elevation oftemperature does not show anisotropy due to crystallographicorientation, and the thermal expansion strain is generated isotropicaly.Therefore, deformation of the ceramic due to strain does not likely tooccur during processing of the ceramic. In addition, when the ceramic isused as an optical transmission medium, deterioration of imagingproperties due to fluctuation of the ambient temperature is not likelyto occur.

Next, a method of producing the above-described Ca—La—F basedtransparent ceramic is explained.

The Ca—La—F based transparent ceramic can be produced by preparing aceramic-forming composition (ceramic raw material composition) includingCaF₂ particles and LaF₃ particles, mixing the ceramic-formingcomposition homogeneously to form a mixture (mixed body) of particles(mixed particles), and sintering the mixture of particles and making themixture a transparent body, wherein the CaF₂ particles and LaF₃particles in the ceramic-forming composition have been preparedseparately.

The above-described production method may include preparation of theceramic-forming composition. Further, the method may include productionof CaF₂ particles and/or LaF₃ particles used in the composition.Alternatively, preliminary prepared CaF₂ particles, LaF₃ particles, or amixture thereof may be used in the ceramic-forming composition.

Here, the ceramic-forming composition denotes a material which may beused in formation of a ceramic through appropriate treatment such asheating, pressurizing, or the like, and includes sinterable CaF₂particles and LaF₃ particles. It is preferable that the CaF₂ particlesand LaF₃ particles included in the composition are highly pure particlesin which content of foreign components is suppressed.

The CaF₂ particles and the LaF₃ particles included in the compositionare produced separately (individually). For example, LaF₃ particlesproduced by a process of producing CaF₂ and LaF₃ simultaneously are notused in the embodiment of the present invention. As an example of aknown method, CaF₂ and LaF₃ are formed simultaneously by adding afluorine compound to a mixed solution containing both of a calciumcompound and a lanthanum compound. However, in accordance with such amethod, it is difficult to obtain fine particles of excellentsinterability having a desired purity and desired grain size (particlediameter).

Preferably, the CaF₂ particles used in the ceramic-forming compositionare fine particles having a maximum grain size (particle diameter) of 5μm or less, preferably 3 μm or less, more preferably 1 μm or less. It isalso preferable that the LaF₃ particles are fine particles having amaximum grain size of 5 μm or less, preferably 3 μm or less, and morepreferably 1 μm or less. Where a particle having a grain size exceeding5 μm is included in the composition, portions of different Laconcentration likely to occur locally in a ceramic after sintering thecomposition. Raw materials of the composition may include coarseparticles having a particle size larger than the above-described upperlimit if the coarse particles are subsequently refined to be not largerthan the upper limit during preparation process of a mixed body ofparticles.

The above-described fine particles include agglomerates (secondaryparticles) composed of a plurality of primary particles agglomerated toeach other. During the preparation of the mixture of particles, it ispreferable to refine the secondary particles and increase the proportion(fraction) of primary particles so as to mix CaF₂ and LaF₃homogeneously.

Preferably, the grain size of primary particles of CaF₂ particles is 200nm or less. Preferably, the grain size of primary particles of LaF₃particles is 200 nm or less. In the description of the embodiment of thepresent invention, “primary particle” denotes a particle which is not anagglomerate of finer particles, and “secondary particle” denotes aparticle constituted by aggregation of a plurality of primary particles.

Preparation of CaF₂

CaF₂ particles may be prepared by making a calcium compound and afluorine compound react in an aqueous solution, and subsequently heatingthe reactant at 100° C. or higher and 300° C. or lower in a closedcontainer.

It is possible to select the calcium compound used in preparation ofCaF₂ particles from organic salts of calcium such as acetate, lactate,oxalate, ascorbate, alginate, benzoate, carbonate, citrate, gluconate,pantothenate, saicylate, stearate, tartrate, glycerinate, andtrifluoroacetate. Inorganic salts of calcium such as chloride,hydroxide, nitrate and sulfate may also be used. Specifically, calciumacetate is preferably used as the calcium compound. Calcium acetate ispreferred because of its high solubility in water. In addition, residualimpurity ions which may occur in the case of using, for example, sulfateor chloride are not likely to occur when the calcium acetate is used asthe calcium compound.

Hydrofluoric acid or the like may be used as the fluorine compound.Hydrofluoric acid is preferably used as the fluorine compound sinceresidual impurity ions are not likely to occur.

The calcium compound and the fluorine compound may be subjected tomutual reaction by preparing aqueous solutions of each compounds bydissolving each compounds in water, and gradually pouring the aqueoussolution of a fluorine compound to the aqueous solution of a calciumcompound.

Preferably, concentration of foreign (other) ions such as La ions iscontrolled to as low as possible in the reaction mixture during thereaction. Where different ions other than Ca ions and fluorine ionscoexist in the reaction mixture, capture of such ions in crystallizingCaF₂ particles results in deterioration of crystallinity of CaF₂, andthe particles tend to be agglomerated. As a result, stronglyagglomerated particles are not dissociated during the below-describedsintering, resulting in low density due to residual voids in theceramic, and the siterability tends to deteriorate. During sintering,small fluctuation of sintering conditions often results in failure toachieve a sintered body of high density. Since the presence of foreignions affect sinterability sensitively and deteriorates thesinterability, it is preferable to suppress the amount of foreign ionsto be as low as possible.

During the reaction, it is preferable to control the amount of fluorinecompound to be larger than the chemical equivalent to the amount ofcalcium compound (chemical equivalent calculated to form CaF₂) so as toimprove the crystallinity of the particles and suppress agglomeration ofthe particles.

Preferably, pouring of the aqueous solution of fluorine compound isperformed while agitating the mixed solution, and the agitation iscontinued after the pouring so as to suppress agglomeration of primaryparticles of CaF₂ crystals being formed. Where particles of stronglyagglomerated state are formed, the agglomerated state cannot bedissociated by pressurizing and heating during sintering and formationof a transparent body, resulting in residual voids, and failing toachieve a dense ceramic. Therefore, it is preferred to perform theagitation sufficiently during the formation of CaF₂ crystals.

After the above-described reaction of calcium compound and fluorinecompound under the normal temperature and normal pressure, it ispreferable to install the reaction mixture in a closed container, andperform hydrothermal reaction treatment to heat and pressurize thereaction mixture at a temperature of 100° C. or higher and 300° C. orlower.

The reaction of the calcium compound and the fluorine compound does notproceed sufficiently only by making them react under normal temperatureand normal pressure, and the crystals having numerous fluorine-loss areformed. Therefore, crystals in the obtained reaction mixture have anon-stoichiometric composition in which proportion of F compared to oneCa is smaller than 2, resulting in low-crystallinity and tendency toagglomeration.

Therefore, it is preferable to further perform the above-describedhydrothermal treatment to complete the reaction of the calcium compoundand the fluorine compound. The container used in the hydrothermaltreatment is not specifically limited. For example, it is possible touse a closed container such as an autoclave made of Teflon (registeredtrade mark). The preferable treatment temperature is 120 to 180° C. Thepreferable pressure is 0.2 to 1.0 MPa which corresponds to a saturatedvapor pressure of water at that temperature range.

By this treatment, it is possible to control the proportion of F to oneCa to substantially 2, and form CaF₂ particles having highcrystallinity. Therefore, it is easy to reduce cohesive force of theparticles. As a result, it is possible to obtain CaF₂ particlesexcellent in sinterability such that the particles are sintered to ahigh density body at relatively low temperature conditions.

According to the above-described method, for example, it is possible toobtain CaF₂ particles including primary particles having an averagegrain size of 100 to 200 nm.

Preparation of LaF₃ Particles

On the other hand, LaF₃ particles may be prepared by way substantiallysimilar to the preparation of CaF₂, by making a lanthanum compound and afluorine compound react in an aqueous solution, and heating at atemperature of not lower than 100° C. and not higher than 300° C. in aclosed container.

Like in the case of calcium compound, a lanthanum compound may beselected from an organic salt of lanthanum, an inorganic material, orthe like. For example, an organic salt of lanthanum such as acetate,lactate, oxalate, ascorbate, alginate, benzoate, carbonate, citrate,gluconate, pantothenate, salicylate, stearate, tartrate, glycerinate,trifluoroacetate or the like may be used. Inorganic salts of lanthanumsuch as chloride, hydroxide, nitrate or sulfate may also be used.Specifically, lanthanum acetate is preferably used as the lanthanumcompound. Hydrofluoric acid may be used as the fluorine compound.

Firstly, like in the preparation of CaF₂, aqueous solutions of each of alanthanum compound and a fluorine compound are prepared, and the aqueoussolutions are made to react gradually under normal temperature andnormal pressure. Here, a small amount of inorganic acid such as nitricacid may be added to the aqueous solution so as to dissolve thelanthanum compound sufficiently.

During the reaction, it is preferable to reduce the amount of foreignions such as Ca ions in the reaction mixture to be as low as possible.By this control, it is possible to suppress deterioration ofcrystallinity of crystallizing lanthanum fluoride, and improve thesinterability. It is preferable to pour the aqueous solution of fluorinecompound in excessive amount to the aqueous solution of lanthanumcompound (for example, aqueous solution of lanthanum acetate) such thatamount of fluorine compound is larger than chemical equivalent to thelanthanum compound (chemical equivalent calculated LaF₃).

By this treatment, it is possible to suppress deterioration ofcrystallinity of lanthanum fluoride, and reduce cohesive force ofparticles. Further, it is preferable to suppress agglomeration ofprimary particles by continuing agitation of the mixed solution duringand after pouring the aqueous solution of fluorine compound.

After the above-described reaction, it is preferable to complete thereaction of the lanthanum compound and the fluorine compound byinstalling and closing the reaction mixture in the closed container, andperforming a hydrothermal reaction treatment to pressurize and heat thereaction mixture at a temperature of not lower than 100° C. and nothigher than 300° C., and preferably at 120 to 180° C. By this treatment,it is possible to suppress fluorine loss, and improve the crystallinityof LaF₃ particles. As a result, the particles are not likely to beagglomerated. Thus, it is possible to obtain LaF₃ particles which can besintered to a high density body at a relatively low temperature.

According to the above-described method, it is possible to obtain LaF₃particles having an average particle size of 100 to 50 nm.

Each of the thus obtained reaction mixture containing CaF₃ particles andreaction mixture containing LaF₃ particles is a suspension in whichcrystal particles are dispersed in an strongly-acidic aqueous solution.Therefore, slurry or dry powder is obtained by solid-liquid separation,and where necessary, by drying the separated material at a temperaturenot lower than room temperature and not higher than 70° C. under areduced pressure. By separating the strongly-acidic aqueous solution bysolid-liquid separation and optional drying, it is possible to controlthe pH value easily during the subsequent treatment and storage,resulting in easy handling. Further, it is possible to suppresscontamination of impurities from the liquid phase.

More preferably, after the solid-liquid separation of theabove-described suspension containing the reaction mixture, the reactionmixture may be subjected to washing treatment for one or a plurality oftimes. For example, the washing treatment may be performed by pouringwashing solvent such as water to the reaction mixture, subjecting thereaction mixture and the solvent to centrifugal separation, and removingthe supermatant liquid. By this treatment, it is possible to removestrongly-acidic liquid and impurities effectively.

Preparation of Composition

Next, the CaF₂ particles and LaF₃ particles at a state of slurry or drypowder are mixed to form a ceramic-forming composition. Theceramic-forming composition denotes a material which can constitute atransparent ceramic by sintering the composition and making thecomposition transparent. A material containing the above-described CaF₂particles and LaF₃ particles may be accepted as a ceramic-formingcomposition. The particles may be homogeneously mixed in thecomposition. The composition may have a form of powder. Alternatively,the composition may have a form of slurry or the like in which theparticles are dispersed or suspended in a dispersion liquid.

Next, after preparing the above-described ceramic-forming composition,the ceramic-forming composition in a state of slurry or suspension issubjected to wet-mixing. By this mixing, CaF₂ particles and LaF₃particles are mixed as homogeneously as possible. Where the mixing isperformed by wet mixing, CaF₂ particles and LaF₃ particles obtained bythe reaction are not likely to be damaged by excessive stress.

In the above-described wet-mixing, it is preferable to deform anddissociate the agglomerated state of particles and mix the primaryparticles as homogeneously as possible.

CaF₂ particles and LaF₃ particles subjected to wet-mixing may includesecondary particles, that is, agglomerates of plural primary particles,even though agglomeration is suppressed during the production of theparticles, for example, by the above-described treatments. Wherehydrofluoric acid is used as the fluorine compound during the productionof the particles, agglomerates teds to be enlarged because ofstrongly-acidic property of the hydrofluoric acid. In addition,agglomerates tend to be enlarged when the CaF₂ particles and the LaF₃particles are dried after preparation of the particles. For example,even when CaF₂ may be around 150 nm and LaF₃ may be around 70 nm inprimary particle sizes, secondary particles may have a grain size ofabout 10 μm.

Where the CaF₂ particles and LaF₃ particles at a state of secondaryparticles in the forms of large agglomerates are mixed and sintered inthat state, the contact surfaces of the CaF₂ particles and LaF₃particles tend to have small area during sintering. In this case, it isdifficult to cause a sufficient solid-state reaction to occur. As aresult, a CaF₂ crystal phase and LaF₃ crystal phase occurmicroscopically in the texture of Ca—La—F based transparent ceramicobtained as a product, resulting in microscopic heterogeneity incomposition. In addition, heterogeneity in the proportion of Ca and Lamay occur in Ca-rich phase of CLF crystal. As a result, theheterogeneity (inhomogeneous distribution) of a refractive index mayoccur, increasing the subsurface scattering of light transmitting in theCa—La—F based transparent ceramic, deteriorating transmittance, and/orcausing difficulty in achieving desired optical properties such as arefractive index and Abbe's number.

Therefore, it is preferable to dissociate agglomerated state ofparticles, increase the fraction of primary particles by deforming theagglomerated particles (secondary particles), and mix the primaryparticles as homogeneously as possible.

Chemical disaggregation and/or mechanical disaggregation may be used asa method of dissociating the agglomerated state of the secondaryparticles.

In the chemical disaggregation, it is possible to lower the cohesiveforce of the particles and deform the agglomerated state by controllingthe properties of solution in which the CaF₂ particles and LaF₃particles are dispersed. In the mechanical disaggregation, agglomeratedstate can be deformed by providing stress such as shear stress to thesecondary particles of CaF₂ particles and LaF₃ particles.

For example, in one of methods of chemical disaggregation, weakening ofcohesive force and disaggregation of agglomerates may be performed bycontrolling the pH to be at least larger than the neutral value in thefluid (liquid) dispersion in which the CaF₂ particles and LaF₃ particlesare suspended. Where the fluid dispersion is at least at a neutralstate, it is possible to weaken the cohesive force. Preferably, thefluid dispersion is controlled to be alkaline fluid having a pH of notlower than 8.

Since CaF₂ particles and LaF₃ particles are soluble in alkaline liquid(alkaline solution), it is possible to expect a reduction of cohesiveforce of agglomerated particles by slightly dissolving the particlesurface. As the alkaline liquid, a solution of inorganic alkali such assodium hydroxide or potassium hydroxide, or a solution of organic alkalisuch as tetramethylammoniumhydroxide (TMAH) or2-hydroxyethyltrimethylammoniumhydroxide may be used. When an organicalkaline liquid is used, it is possible to avoid incorporation ofimpurities such as sodium or potassium (which may be contained in theinorganic alkaline solution) into the dispersion. In addition, theorganic alkaline liquid is preferable since it is decomposed and removedeasily by heating and is not likely to remain in the Ca—La—F basedtransparent ceramic.

In the above-described chemical disaggregation method, it is possible todrop alkali solution to a fluid dispersion suspending the CaF₂ particlesand LaF₃ particles to control pH. Alternatively, composition of powderor slurry state may be added to a liquid (solution) of controlled pH todisperse CaF₂ particles and LaF₃ particles in the liquid.

On the other hand, as the mechanical disaggregation method, it ispossible to perform agitation (stirring) using an agitating blade or thelike. The mechanical disaggregation may be performed using a dispersersuch as beads-mill, high-pressure homogenizer, high-speed disperser orthe like.

When an excessive stress is provided during the mechanicaldisaggregation, residual stress may remain in the primary particles anddeformation of the particles may occur. As a result, cracking or warpingof a ceramic tends to occur during sintering. Therefore, it ispreferable to agitate at an appropriate speed in the case of using anagitating blade or the like.

In the mechanical disaggregation, it is preferable to disperse particlesusing the high-pressure homogenizer.

It is preferable to use the chemical disaggregation method and themechanical disaggregation method in combination. In this case, it iseasy to dissociate the agglomeration while reducing the stress appliedto the particles to low level.

Preferably, 80% or more (in number fraction), and more preferably, 95%or more of each of the CaF₂ particles and the LaF₃ particles is made toprimary particles by the above-described dissociation of agglomerates.

By the thus dissociating and deforming the agglomerates of the CaF₂particles and the LaF₃ particles, and mixing the particles whiledispersing the particles at a substantial state of primary particles, itis possible to mix the primary particles of CaF₂ and primary particlesof LaF₃ more homogeneously. As a result, it is possible to reduce thecompositional fluctuation, that is, fluctuation of the refractive indexin the ceramic after the sintering, thereby obtaining a transparentceramic having excellent internal homogeneity.

In the above-described wet-mixing, mixing ratio of the CaF₂ particlesand LaF₃ particles may be determined based on at least one of therefractive index and the Abbe's number of the desired Ca—La—F basedtransparent ceramic, since the refractive index and the Abbe's numberhave linear functional correlation with the proportion of Ca and La.

By the above-described wet-mixing, it is possible to prepare a particlemixture (mixed body of particles) at a state where CaF₂ particles andLaF₃ particles are mixed as homogeneously as possible.

After that, the particle mixture in which CaF₂ particles and LaF₃particles are mixed homogeneously in a liquid dispersion is subjected tosolid-liquid separation. Next, the separated particle mixture is dried.For example, the mixture may be dried at a temperature not lower thanroom temperature and not higher than 70° C. while being degassed under areduced pressure.

Preferably, a dried body free of crazing, cracking, voids or the likeare formed by pressurizing (compressing) the mixture using dry uniaxialpress or the like.

Next, a sintered body (precursory sintered body) is prepared bysubjecting the thus obtained dried body to sintering by heating thedried body at a temperature of not lower than 700° C. and not higherthan 1000° C.

At that time, since the agglomerates of CaF₂ primary particles and LaF₃primary particles have been decomposed and CaF₂ particles and LaF₃particles have high crystallinity, it is possible to obtain a sinteredbody of high relative density by the sintering.

If the sintering temperature is lower than 700° C., it is difficult toperform sintering of the dried body. On the other hand, if the sinteringtemperature exceeds 1000° C., loss of fluorine from the crystalstructure becomes remarkable, thereby deteriorating the transparency ofthe ceramic. Therefore, 1000° C. is preferred as the upper limit of thetemperature. The preferable temperature range is not lower than 800° C.and not higher than 900° C.

After that, the sintered body is made transparent by secondary sinteringby heating the sintered body at a temperature of not lower than 700° C.and not higher than 1300° C. while pressurizing the sintered body at apressure of not lower than 500 Kg/cm² and not higher than 3000 Kg/cm² inan inert atmosphere such as argon or nitrogen.

The sintered body may be made transparent, for example, using a hotisotropic press (HIP).

During pressurizing, residual pores in the sintered body are pressedout, and the sintered boy is made transparent and further denser thanthe above-described sintered body (precursory sintered body). As aresult, it is possible to obtain a transparent sintered body having highrelative density. Thus, production of a Ca—La—F based transparentceramic is completed.

During the formation of the above-described precursory sintered body,and during making the sintered body transparent, CaF₂ particles and LaF₃particles occur a reaction (reactive sintering) accompanied by diffusionof Ca and La, resulting in forming polycrystalline body of CLF crystals.Where the temperature at the time of pressurizing and heating is lessthan 700° C., it is difficult to cause a reaction of CaF₂ particles andLaF₃ particles to occur. Therefore, 700° C. is preferred as the lowerlimit of the temperature. On the other hand, where the temperatureexceeds 1300° C., there is a possibility that a liquid phase isseparated from a solid phase, and it is difficult to control the loss offluorine even under pressurized conditions. Therefore, 1300° C. ispreferred as the upper limit of the temperature during making thesintered body transparent. The preferable temperature range is not lowerthan 800° C. and not higher than 1100° C. A more preferable temperaturerange is not lower than 800° C. and not higher than 900° C.

The ceramic produced through the above-described production process is apolycrystalline body including CLF crystals. Where coarse crystals areincluded in the polycrystalline body, there is a possibility thatisotropic thermal expansion is disturbed. Therefore, the grain size ofthe crystals is preferably controlled to be 100 μm or less in thepolycrystalline body constituting the ceramic.

In the above-described method, sintering of the particle mixture to makea transparent body were performed by two steps including a firstsintering to form the precursory sintered body and a second sintering toform a transparent body. Alternatively, the sintering and the formationof the transparent body may be performed in a single apparatus bychanging the temperature and pressure in accordance with a predeterminedschedule.

In the above-described method of producing a Ca—La—F based transparentceramic, CaF₂ particles and LaF₃ particles prepared separately from theCaF₂ particles are mixed to form a particle mixture, and the particlemixture is sintered and becomes transparent. According to such aproduction method, it is easy to ensure a sinterability of the particlemixture. Therefore, it is possible to produce a Ca—La—F basedtransparent ceramic which has an Abbe's number as high as that offluorite and refractive index higher than that of fluorite.

Further, since a sintered body is formed from a particle mixture, andthe sintered body becomes transparent by heating while pressurizing thesintered body in an inert atmosphere, it is possible to densify thesintered body, thereby producing a Ca—La—F based transparent ceramichaving a high transparency.

Since the CaF₂ particles are prepared by making the calcium compound andfluorine compound react in an aqueous solution, and by subsequentlyheating in a closed container at a predetermined temperature, it ispossible to produce CaF₂ particles of high sinterablity.

Since the LaF₃ particles are prepared by making the lanthanum compoundand fluorine compound react in an aqueous solution, and by subsequentlyheating in a closed container at a predetermined temperature, it ispossible to produce LaF₃ particles having high sinterablity.

Since the particle mixture is produced by wet-mixing of CaF₂ particlesand LaF₃ particles, excessive stress is not likely to act on primaryparticles of CaF₃ particles and LaF₃ particles. As a result, crackingand warping are not likely to occur at the time of sintering.

Specifically, chemical disaggregation can be enhanced and stress on theprimary particles can be suppressed by wet-mixing of CaF₂ particles andLaF₃ particles in an alkaline liquid.

Further, by using organic alkaline liquid as the alkali solution,components of the alkaline liquid do not likely remain in the ceramicafter the sintering, thereby making it easy to ensure the quality of theCa—La—F based transparent ceramic.

In addition, since the wet-mixing is performed using a mechanical mixingdevice, it is easy to perform sufficient disaggregation.

Since the mixing is performed while controlling the mixing ratio of CaF₂particles and the LaF₃ particles based on at least one of desiredrefractive index and Abbe's number, it is easy to produce a Ca—La—Fbased transparent ceramic having a desired refractive index and Abbe'snumber.

EXAMPLES

Next, Examples according to embodiments of the present invention areexplained.

Example 1 Preparation of CaF₂ Particles

Distilled water of 640g was added to a calcium acetate hydrate of 180.4g (1 mol). An aqueous solution of calcium acetate was prepared bycompletely dissolving the hydrate.

163.8g (4 mol) of hydrofluoric acid of 50% in HF concentration wasprepared. The same weight of distilled water was added to thehydrofluoric acid to form an aqueous solution of hydrogen fluoride.

The aqueous solution of hydrogen fluoride (the diluted hydrofluoricacid) was added slowly to the aqueous solution of calcium acetate whileagitating the aqueous solution of calcium acetate by rotating a bladestirrer (10 cm in blade diameter) at 300 rpm. An inlet of the aqueoussolution of hydrogen fluoride was provided to a side wall of a plasticbeaker (13 cm in diameter) containing the aqueous solution of calciumacetate, and the aqueous solution of hydrogen fluoride was drawn by aroller pump and was poured to the aqueous solution of calcium acetatefor about 1 hour.

After completion of pouring of the aqueous solution of hydrogenfluoride, agitation was continued for 6 hours. Thus, by reducing theparticle size while deforming the agglomerated particles, a CaF₂ slurrywas prepared.

The thus obtained CaF₂ slurry was contained and closed in an autoclavemade of Teflon (registered trade mark), and subjected to hydrothermalreaction by heating and pressurizing the slurry at a temperature of 145°C. for 24 hours. Thus, preparation of a slurry suspending CaF₂ particleswas completed.

Preparation of LaF₃ Particles

LaF₂ particles were prepared in accordance with a similar method as thatused in the preparation of CaF₂ particles.

Distilled water of 1000g was added to a lanthanum acetate hydrate of290.4 g. Further, nitric acid was added and the lanthanum acetatehydrate was completely dissolved. Thus, an aqueous solution of lanthanumacetate was prepared.

183.8 g (5 mol) of hydrofluoric acid of 50% in HF concentration wasprepared. The same weight of distilled water was added to thehydrofluoric acid to form an aqueous solution of hydrogen fluoride.

Like in the case of preparation of CaF₂ particles, aqueous solution ofhydrogen fluoride was slowly poured in the aqueous solution of lanthanumacetate while agitating the aqueous solution of lanthanum acetate. Aftercompletion of pouring, agitation was continued, reducing the particlesize while deforming the agglomerated particles, thereby preparing aLaF₃ slurry.

Further, the thus obtained LaF₃ slurry was subjected to a hydrothermaltreatment in a similar manner as in a treatment of CaF₂, therebycompleting the preparation of a slurry dispersing LaF₃ particles.

FIG. 1A and FIG. 1B respectively shows a transmission electronmicroscopic (TEM) photograph of the thus obtained CaF₂ particles andLaF₃ particles. The diameter of primary particle was about 150 nm inCaF₂ and about 70 nm in LaF₃. Based on the observation of lattice imagesin the particles under high magnification observation using the TEM,excellent crystallinity was confirmed in both of the CaF₂ particles andLaF₃ particles. Under the low-magnification observation, both of theparticles show formation of secondary particles by agglomeration of aplurality of primary particles. The secondary particles had a diameterof at most about 10 μm.

Wet Mixing

The thus prepared slurry suspending CaF₂ particles and the slurrysuspending LaF₃ particles were weighed such that molar ratio of La/Cawas 0.3, and were mixed to each other, thereby forming a ceramic-formingcomposition.

The ceramic-forming composition was subjected to centrifugal separation.After removing the supermatant liquid, the composition was rinsed by twotime operations of adding distilled water and centrifugal separation soas to remove the hydrogen fluoride and nitric acid as much as possible.

After that, the sediment (precipitate) obtained by the centrifugalseparation was dispersed in a distilled water, and TMAH was added to thewater to control pH to be 13. Then a particle mixture was prepared byperforming agitation for 20 hours using agitating blade to disaggregatethe agglomerated particles while chemically dissolving the surfaceportions of fluorides, and wet-mixing the particles homogeneously.

Sintering

A powder was obtained by drying the particle mixture at a temperature of100° C. The dried powder of 6 g was formed to a compact by uniaxialpress molding using a mold having a diameter of 30 mm. White coloredsintered body was obtained by sintering the compact by heating it for 1hour at a temperature of 800° C. in an air atmosphere.

Formation of Transparent Body

Next, using a hot isotropic press (HIP) apparatus (Dr. HIP, registeredtrade mark, provided by Kobe Steel, Ltd.), the sintered body was heatedat 1100° C. for 2 hours while an isotropic pressure of 1500 Kg/cm² wasapplied in an argon atmosphere. By this treatment, closed pores remainin the interior of the sintered body were pressed out from the sinteredbody, and the sintered body became transparent. Thus, a Ca—La—F basedtransparent ceramic was obtained.

Examples 2, 3

Ca—La—F based transparent ceramics were obtained in a similar manner asin Example 1, while the molar ratio of La/Ca was changed to 0.1 and 0.4in the wet mixing.

Example 4

A Ca—La—F based transparent ceramic was obtained in a similar manner asin Example 1, whereas TMAH was not added in the wet mixing.

The Ca—La—F based transparent ceramic obtained in Examples 1 to 4 weresubjected to the below described measurements.

CLF Crystal

FIG. 2 and Table 1 show a result of X-ray diffractometory of the Ca—La—Fbased transparent ceramic (Ca_(0.7)La_(0.3)F_(2.3)) obtained in Example1.

In addition, generally known data of CLF crystal(Ca_(0.65)La_(0.35)F_(2.35)) described in a JCPDS card is shown in FIG.2B and Table 1.

TABLE 1 Example 1 Ca_(0.65)La_(0.35)F_(2.35) Relative Relative 2θ(deg)d(A) intensity hkl 2θ(deg) d(A) intensity 27.5336 3.23693 100.00 11127.341 3.25926 100.0 31.8928 2.80608 11.11 200 31.674 2.82260 21.745.7104 1.98489 60.81 220 45.405 1.99588 74.5 54.1780 1.69298 26.25 31153.816 1.70209 38.6 56.7631 1.62186 5.76 222 56.417 1.62963 4.5 66.61341.40395 2.85 400 66.160 1.41130 6.8 73.5008 1.28848 7.55 331 72.9941.29510 11.0 75.7368 1.25591 1.92 420 75.212 1.26231 4.8 84.4934 1.146674.69 422 83.899 1.15232 10.7

When the X-ray diffraction pattern of the Ca—La—F based transparentceramic (Ca_(0.7)La_(0.3)F_(2.3)) of Example 1 is compared with theX-ray diffraction pattern of CLF crystal (Ca_(0.65)La_(0.35)F_(2.35))shown in JCPDS01-087-0975, both patterns are substantially consistent toeach other. A slight difference in peak position is caused by a slightdifference in lattice constants due to different La/Ca ratios.Therefore, based on the comparison of the both X-ray diffractionpatterns, it was confirmed that the Ca—La—F based transparent ceramicobtained in Example 1 was composed of CLF crystals or contained CLFcrystals, and was not a mere dense sintered body composed of CaF₂particles and LaF₃ particles.

Evaluation of Chemical Disaggregation

FIG. 3 shows a result of measurements of transmittance of the CLFtransparent ceramic (2 mm thick) obtained in Example 1, andtransmittance of the CLF transparent ceramic (2 mm thick) obtained inExample 4. A in FIG. 3 denotes a result of Example 1 in which TMAH wasadded in the wet-mixing. B denotes a result of Example 4 in which TMAHwas not added in the wet-mixing.

By adding TMAH and performing wet-mixing, transmittance was improvedthroughout the entire wavelength range, and the transmittance of lightof 550 nm in wavelength was improved from 58% to 72%. By this result, itwas confirmed that agglomerated particles were deformed by alkalineliquid, and compositional fluctuation in the interior of the CLFtransparent ceramic was reduced.

Refractive Index

Table 2 show results of measurements of refractive indexes (ng, nF, nd,ne, nF, nC) for Fraunhofer's g-line, F-line, a-line, d-line, and C-linein CLF transparent ceramics of Examples 1 to 3 which were produced whileusing a different mixing ratio of CaF₂ particles and LaF₃ particles suchthat La/Ca was 0.1, 0.3, and 0.4 respectively. The measurements ofrefractive indexes were performed using a refractometer PR-2 made byCarl Zeiss Jena.

TABLE 2 La/Ca nC nd ne nF ng Example 1 0.3 1.4395 1.4952 1.4966 1.49911.5021 Example 2 0.1 1.4533 1.4548 1.4560 1.4582 1.4609 Example 3 0.41.5116 1.5134 1.5148 1.5175 1.5207Correlation between Mixing Ratio of Particles and Abbe's Number

FIG. 4 shows the relationship between the refractive index (nd), theAbbe's number and La/Ca in Examples 1 to 3 which were produced whileusing a different mixing ratio of CaF₂ particles and LaF₃ particles suchthat the La/Ca was 0.1, 0.3, and 0.4 respectively.

In the results, the refractive index tended to increase whereas theAbbe's number tended to decrease with increasing addition of LaF₃particles. The refractive index was increased from 1.43 (a refractiveindex of fluorite in which La/Ca is 0) to 1.52 in accordance withincreasing La/Ca from 0 to 0.4. Although the Abbe's number was decreasedto 87 at La/Ca =0.4, this number still remains in a low dispersionregion. Thus, novel transparent materials having optical propertiespreviously unknown, for example, low-dispersion and high refractiveindex, were obtained where La/Ca was larger than 0 and not larger than0.4.

Anomalous Partial Dispersion Ratio

The partial dispersion ratio was calculated based on the refractiveindexes measured for respective lines. Correlation between the Abbe'snumber and the partial dispersion ratio is shown in FIG. 5 along withthe correlation in various optical glasses (normal glasses). Soliddiamonds in the figure show correlation of the various optical glasses.

The partial dispersion ratio (Pg, F) was calculated using thebelow-described formula (1) based on the refractive indexes forFraunhofer's g-line, F-line, and C-line.

Pg,F=(ng-nF)/(nF-nC)   (1)

The Abbe's number (vd) is calculated by the following formula (2)

vd=(nd-1)/(nF-nC)   (2)

The partial dispersion ratios of various optical glasses are plottedsubstantially along a straight line, whereas data of all of three CLFtransparent ceramics obtained in Examples 1 to 3 were largely deviatedfrom this straight line. Thus, anomalous partial dispersion propertieswhich cannot be seen in various optical glasses was confirmed.

Especially, the partial dispersion shows the largest deviation at La/Caof 0.1, and a strong effect of reducing a secondary spectrum can beobtained, Since the secondary spectrum has a large influence in atelescopic lens of long focal length, utilization of the CLF transparentceramic is effective in reducing chromatic aberration.

Comparative Example

A ceramic was produced in a similar manner as in Example 1 while usingcerium acetate instead of the lanthanum acetate. As a result, a ceramicof transparent property could not be obtained.

Comparative Example 2

A ceramic was produced in a similar manner as in Example 1 while usingyttrium acetate instead of the lanthanum acetate. As a result, a ceramichaving a transparent property could not be obtained.

Comparative Example 3

A ceramic was produced in a similar manner as in Example 1 while usingparticles obtained by mixing calcium acetate and lanthanum acetate andmaking the mixture react with hydrofluoric acid instead of using CaF₂particles and LaF₃ particles prepared separately. As a result, a ceramicof transparent property could not be obtained.

Example 5

A particle mixture at a slurry state obtained according to Example 1 wasprepared as a starting material. As described-above, deformation ofagglomerated particles or a weakening of cohesive force were performedin the starting materials by agitation for 20 hours in a state of pH 13.Further, the agglomerated particles were mechanically deformed by usinga high-pressure homogenizer (Nanomizer, registered trademark of S.G.Engineering Inc., provided by Yoshida Kikai Co. Ltd.). The particlemixture was controlled such that total weight of CaF₂ and LaF₃ in theslurry was 20 wt. %, and was subjected to treatment for 20 times in theabove-described high pressure homogenizer at a pressure of 200 MPa. Thetreated slurry was subjected to a centrifugal separation, and was driedat 100° C. after removing the supermatant liquid, thereby obtaining apowder. This mixed-powder was subjected to sintering and was made atransparent body in a similar manner as in Example 1, thereby obtainingCa—La—F based transparent ceramic, where formation of a transparentbody, that is, secondary sintering was performed at a temperature of900° C. FIG. 6 shows the transmittance in the thus obtained transparentceramic (2 mm thick). The transmittance for a light of 550nm inwavelength was 72% in Example 1, and was improved to 79% in Example 5 inwhich the particle mixture was treated using the high pressurehomogenizer. By this result, it was confirmed that transmittance oflight in a ceramic obtained by sintering a particle mixture and forminga transparent body could be improved by using both of a weakening ofcohesive force by a chemical treatment and mechanical disaggregation. Itis understood that compositional fluctuation in the sintered body wasreduced since the agglomerated particles in the particle mixture weredispersed to a state of primary particles. Further, since thesolid-state reaction between primary particles of CaF₂ and LaF₃ wasenhanced, and the sintered body could become transparent even though thetemperature of secondary sintering was reduced from 1100° C. to 900° C.,fluorine loss was reduced in the sintered body.

As described above, according to some embodiments of the presentinvention, it is possible to provide a Ca—La—F base material having ahigh Abbe's number like as fluorite and refractive index higher thanfluorite as a transparent ceramic. Since the above-described transparentceramic shows anomalous partial dispersion compared with general opticalglass, it is possible to materialize an optical system of excellentoptical properties by using the transparent ceramic as an opticalelement. An embodiment of the present invention also provides a methodof producing the transparent ceramic, and a composition used in theproduction method. Therefore, embodiments according to the presentinvention have high industrial applicability.

While embodiments of the invention have been described and illustratedabove, it should be understood that these are exemplary of the inventionand are not to be considered as limiting. Additions, omissions,substitutions, and other modifications can be made without departingfrom the scope of the present invention. Accordingly, the invention isnot to be considered as being limited by the foregoing description, andis only limited by the scope of the appended claims.

1. A transparent ceramic that is constituted of a polycrystalline body including crystals of (Ca_(1−x)La_(x))F_(2+x), where x denotes a number larger than 0 and not larger than 0.4, and has a transparency capable of transmitting light, wherein transmittance of light of 550 nm in wavelength is not lower than 50% and the transparent ceramic has a thickness of 2 mm.
 2. A Ca—La—F based transparent ceramic according to claim 1, wherein a refractive index is not smaller than 1.43 and not larger than 1.55, and an Abbe's number is not lower than 80 and not higher than
 95. 3. A Ca—La—F based transparent ceramic according to claim 1, wherein an Abbe's number is not lower than 80 and not higher than 95, and a partial dispersion ratio Pg, F shown by the below-described formula (1) is not lower than 0.53 and not higher than 0.56. Pg, F=(ng-nF)/(nF-nC)   (1), where each of ng, nF, and nC denotes a refractive index with respect to Fraunhofer's g-line, F-line, and C-line respectively.
 4. An optical element which comprises a Ca—La—F based transparent ceramic according to claim 1 and is shaped to have a predetermined shape.
 5. An optical system comprising at least a pair of a convex lens and a concave lens in an optical path, wherein one of the convex lens or the concave lens is made of the Ca—La—F based transparent ceramic according to claim 1 and the other lens is made of a material different from the Ca—La—F based transparent ceramic. 